If Excel replaces existing characters instead, you need to press the Insert key to get out of overtype mode (in which the new characters you input eat up the existing ones on the right) before you start inputting.

The second is the same sentence with a random letter substitution (after making a copy of the sentence, I put the cursor at the start of the sentence, closed my eyes, held down the right-arrow key for a second, then, still with my eyes closed, pressed a random key on the keyboard in overtype mode).